The telecommunications carrier industry began to develop a concept called "Broadband Integrated Services Digital Network" or B-ISDN. This was conceived as a carrier service to provide high speed communications to end users in an integrated way.
Different techniques have been developed for transporting information over a network, such as packet switching techniques whereby the digitized data are arranged into so-called bit packets, and circuit switching techniques. In packet switching, the bit packets may either be of fixed length (like in the so-called Asynchronous Transfer Mode (ATM) where the packets, also called cells, are all of a conventional fixed length), or be variable length.
The basic advantage of packet switching techniques over circuit switching techniques, is to allow a statistical multiplexing of different types of data over a link, which optimizes the transmission bandwidth. The present invention shall try to keep this advantage under various circumstances. Although the present invention applies to all kinds of packet switching techniques, including Frame Relay, the technology selected here as an example made to illustrate the invention and show how to deliver the B-ISDN service, in the detailed implementation, is the "Asynchronous Transfer Mode" or ATM.
The almost universal acceptance of ATM comes from the fact that ATM is a compromise. ATM does handle all the different kinds of communication traffic, including voice, data, image, video, high quality sound and many others, and it can be used in both the LAN (Local Area Network) and the WAN (Wide Area Network) network environments.
ATM does not handle voice as efficiently as does an isochronous network, it does not either handle video as easily as isochronous transfer does, it may not always handle data as effectively or efficiently as a Packet Transfer Mode or Frame Relay system do, and it is likely to be problematic in any high error rate environment, but it however offers a tremendous advantage over other networks since it enables combining all these kind of multimedia data for transport over the same network. This means that instead of having a proliferation of many specialized kinds of equipment for different functions we can have a single type of equipment and network which will do everything.
The basic key concepts of ATM are as follows:
All informations (voice, image, video, data . . . ) are transported through the network in very short, fixed length (48 data bytes plus a 5 byte header) blocks called "cells", (herein also referred to as ATM cells).
The information flow along paths, called "virtual channels" (VC) set up as a series of pointers through the network. The cell header contains an identifier that links the cell to the correct path towards its destination. Cells on a particular virtual channel always follow the same path through the network and are delivered to the destination in the same order in which they are received.
ATM is designated so that simple hardware based logic elements may be employed at each node to perform the switching. On a link of 1 Gbps a new cell arrives and a cell is transmitted every 0.43 microsec. This leaves little time to decide what to do with an arriving cell.
At the edges of the network, i.e. on a network port or an access node, user data frames or packets are broken up into cells. Continuous data streams such as, for instance, voice and video are assembled into ATM cells. At the destination side of the network, the user data frames are reconstructed from the received cells and forwarded to the end user in the form that they were delivered to the network. This adaptation function is considered part of the network but is in a higher layer function, which is called ATM Adaptation Layer (AAL). These edge or port equipments are already operative and well defined by International Standards perfectly specifying, for instance, so-called ATM Adaptation Layers (AALS).
The ATM cell switching network only checks cell headers for errors and simply discards cells in error. The adaptation function AAL is external to the switching network and depends somewhat on the type of traffic, but for data traffic it usually checks for errors in data frames received and in case of error found, then it discards the whole frame. At no time does the ATM network attempt to recover from errors by the re-transmission of information. This function is up to the end user devices and depends on the type of traffic being carried and the adaptation layer being used. For instance, end user equipments have been specified for recovering discarded voice data through so-called interpolation/ extrapolation techniques.
Information about ATM networks can be found for example in "High Speed Networking Technology: An Introductory Survey", June 1993, Document Number GG24-3816-01, IBM International Technical Support Center, Raleigh.
Information on the Standards of ATM can be found in the International Telecommunication Union (ITU) Recommendations.
But, as mentioned above, Asynchronous Transfer Mode (ATM) may be considered as lacking efficiency on a number of situations. For instance, when applying to voice transportation, whereby a Private Branch Exchange (PBX) serving a number of telephone sets is attached to a Voice Server which both codes and compresses the digitized user's voices. In this case, 160 bytes long data frames (or packets) may be compressed by a factor 5 when applied to Global System for Mobile (GSM) telephony, or even be compressed by a factor 8. Accordingly, the data packets to be transported over ATM cells (a 48 data bytes long payload+a 5 bytes long header) shall bear 32 bytes or 20 bytes payload data. Therefore, one data packet provided to the ATM network port would not fill-up an ATM cell payload field.
In other words either one pads each ATM cell with dummy bytes (say 16 or 28 bytes per cell) or one aggregates user's frames into several consecutive ATM cells. One should note that the so-called byte is referred to as "octet" in the International Telecommunication Union standards terminology. From an efficiency standpoint, the first solution should obviously be avoided as it leads to inefficient bandwidth occupancy.
This leaves us with the second solution.
One may easily imagine the various problems raised for handling in a conventional ATM network such multiple frames (or packets), particularly when they are of variable length like those issuing from a voice server performing silence removal operations, and/or shorter than a conventional ATM cell payload, be they provided to the ATM network directly or through another network (e.g. a Packet Switching Access Network). On the other hand, one should bear in mind that Standards requirements on ATM Adapter Layer (AAL) may then have to be redefined that enable performing the adequate encapsulation and transport of user data packets, be they fixed or variable length, onto an ATM cell stream. Accordingly, some of the existing networks hardware become useless, which, obviouly raises a number of serious problems from a practical standpoint.
The current International Telecommunication Union (ITU) standards do not specifically address the problem of aggregating composite short packets which could be of variable length, within an ATM cell in such a manner that the ATM Adaptation Layer at the receiving end of the network can easily retreive the individual packets from the incoming ATM cell stream.
Various solutions have been proposed that recommend creating a new standardized AAL thus implying in addition to the hassle of modifying the standards, also modifying the currently used hardware.